Most conventional thermostatic control systems monitor room temperature to determine when to actuate or deactuate heater power. After sunrise, heat flows into a building due to solar loading and the total amount of heat flow is influenced by increasing outdoor temperature. These systems do not account for the influence of radiant and conductive heat flow through outside walls and windows on room temperature. Therefore, the building furnace may be providing heat despite the thermal energy entering the building. This defect in existing thermostat systems results in excessive furnace "on time" because the furnace could be shut off earlier as the entering thermal energy will continue to raise the internal temperature to the desired threshold level. Instead the furnace stays on until the threshold level is reached. The entering thermal energy continues to heat the room after the threshold level is reached causing room temperature "overshoot".